THE INTERPHASE NUC1.EUS 



methods of fixation, they assert, precipitate the Feulgen-positive 

 material into artefactual aggregates, though the original disperse condi- 

 tion can be maintained by suspension of the nuclei in lo per cent 

 sucrose. 



Heterochromatic granules — In nuclei of the mouse in tissue culture. 

 Fell and Hughes^ have shown that the heterochromatic granules 

 which are Feulgen-positive after fixation can also be identified by 

 phase-contrast in the living nucleus, in the same position and form. 

 The chromonemata are also faintly Feulgen-positive, and some threads, 

 slightly denser than the rest, can be recognized in the living nucleus. 

 It is possible to maintain that the entire Feulgen-positive charge on 

 such heterochromatic granules and chromonemata is precipitated 

 thereon during fixation and that their contrast in life is due to their 

 constituent proteins or perhaps to lipoidal material in addition, but if 

 this were true, then the living nucleus should show no detail when 



Figure 9 Blastomere nucleus of Fundulus 

 heteroclitus in early prophase. Chromatin 

 condensing on walls of chromosomal 

 vesicles (x 1800 + ) . From Richards^* 

 {By courtesy, Biol. Bull.). 



photographed at 2,600 A, except for the nucleoli. Now in the ultra- 

 violet photomicrographs of sarcoma cells published by Ludford et 

 alii,^^ and by Ludford and Smiles^^ it is clear that absorbing granules 

 smaller than the nucleoli are abundantly present within the nucleus, 

 (Plate III (7)), which has a very similar appearance both in the ultra- 

 violet and by phase-contrast. On the other hand, the ultraviolet photo- 

 micrographs of isolated nuclei suspended in sucrose which Ris and 

 Mirsky^* have published show no internal structure, and suggest that 

 the nucleoli and perhaps other constituents have been dissolved as a 

 result of the prpcedure to which they have been submitted. Although 

 Brown, Callan and Leaf^^ found no trace of nucleic acid in the 

 nuclear sap of Xenopus oocytes, one would not expect the same neces- 

 sarily to be true of somatic nuclei; for their structure is very diflferent. It 

 may well be that there is no uniform state of aggregation of the DNA in 

 the interphase nucleus. 



DNA content in nuclei — It would probably not be realized from the 

 inspection of stained preparations that the amount of DNA in the 

 nucleus greatly outweighs that of RNA, for the nucleoli seem at least as 

 prominent as the Feulgen-positive material. However the analyses of 



33 



